1. Field of the Invention
This invention relates to styrene polymer sheet and more particularly to low density styrene polymer sheet material.
Over the past few years there has been a great increase in the use of expanded styrene polymer in sheet form. One of the main reasons for the increased demand for this material is that expanded styrene polymer, even in thin sheet form, offers the advantages of high structural strength and low density. Because of its excellent physical properties and relatively low cost expanded styrene polymer sheet material is used in the manufacture of thermoformed products such as egg cartons, trays used in the retail packaging of meat and produce, and in various other commercial and industrial applications.
In order to produce expanded styrene polymer sheet which has good physical strength properties it is desirable to use a feedstock which has been specially formulated for extrusion into sheet material. Suitable feedstock contains, in addition to an expanding agent, a nucleating system to facilitate the production of microporous expanded styrene sheet material. Fine-celled expanded styrene polymer sheet has been found to have greater structural strength and less brittleness than larger celled expanded sheet material and is, therefore, more readily adaptable to post extrusion procedures such as the above-mentioned thermoforming operations.
Manufacturers of expanded styrene polymer sheet have found that they can reduce the cost of production of expanded styrene polymer sheet by combining or replacing extrusion grade feedstock with various other styrene polymer material, such as a general purpose styrene polymer and off-grade expandable styrene polymer particles, e.g., beads or pellets which are unsuitable for molding purposes because they are over or under sized or because they contain an insufficient amount of blowing agent for the production of high quality molded products. These materials can be made useful for the extrusion manufacture of expanded sheet material by adding nucleating agents to them and, if desired, incorporating additional blowing agent into them, for example, by injecting blowing agent into molten feedstock made from these materials while it is resident in the barrel of the sheet extruder.
One of the problems encountered in the production of extruded styrene polymer foamed sheet material is that the product sometimes lacks uniformity in flexibility and physical appearance. Thus, while some portions of an extruded sheet product are of high quality, other portions contain large and irregularly sized cells. Large cells in foamed polymer sheet not only cause the sheet to be less desirable in appearance but also render it quite brittle so that it cannot satisfactorily be used in thermoforming operations and other product applications.
It has been determined that a significant cause of the development of large cells in extruded foam sheet product is the presence of contaminants and/or moisture in the feedstock. Contaminants are often contained in the feedstock because chemicals and other substances which were added to the styrene polymer polymerization formulation during or subsequently to polymerization, such as suspending agents or other polymerization aids, were not completely removed from the polymerized product. Moisture in the polymer feedstock often results from the sorption of water droplets or vapor onto or into the feedstock. Moisture may also be introduced into the molten polymer by chemical reaction between the components of the nucleating system in the heated barrel of the extruder.
It is believed that the variation in the distribution of the contaminants and moisture throughout the feedstock is responsible for the wide variation in cell size in the extrudate. For example, in expandable polymer materials which contain a nucleating system comprised of an acid and an alkali metal carbonate which react and release carbon dioxide, high local concentrations of contaminants are likely to interfere with the reaction rate of the nucleating system thereby causing uneven rates of release of carbon dioxide with the result that the expanded product may contain cells which vary widely in size. The variation in distribution of contaminants and moisture in the feedstock is often caused by the use of a feedstock comprised of a mixture of various batches of materials, some of which contain considerable amounts of contaminants and/or moisture. Although the variation can be minimized by blending operations, this is not usually done because the additional expense of such operations makes it economically unfeasible. The moisture content of the extruder feedstock can be controlled by selecting moisture-free feed materials or by drying moisture-containing feedstock but this would also prohibitively increase the cost of producing a competitive expanded styrene polymer sheet. It would be desirable to eliminate the effects caused by contaminants and moisture in sheet extruder polymer feedstock without resorting to unnecessary blending and/or drying steps.